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    <div class="post-body" itemprop="articleBody"><h2 id="std-shared-ptr"><a href="#std-shared-ptr" class="headerlink" title="std::shared_ptr"></a>std::shared_ptr</h2><p><a target="_blank" rel="noopener external nofollow noreferrer" href="https://en.cppreference.com/w/cpp/memory/shared_ptr">std::shared_ptr</a>共享智能指针，可以支持多个智能指针共享一个对象实例。shared_ptr持有的实例会在下面场景下被销毁：</p>
<ul>
<li>持有实例的最后一个shared_ptr被销毁；</li>
<li>持有实例的最后一个shared_ptr被另一个智能指针赋值，或者shared_ptr调用了reset();</li>
</ul>
<p><img src="/images/C++/shared_ptr.drawio.svg" alt="shared_ptr"></p>
<p>使用方法：</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line">shared_ptr&lt;User&gt; sp1 = <span class="built_in">make_shared</span>&lt;User&gt;(<span class="number">1</span>, <span class="string">&quot;sp1&quot;</span>, <span class="string">&quot;123456&quot;</span>);</span><br><span class="line"><span class="function">shared_ptr&lt;User&gt; <span class="title">sp2</span><span class="params">(<span class="keyword">new</span> User(<span class="number">2</span>, <span class="string">&quot;sp2&quot;</span>, <span class="string">&quot;123456&quot;</span>))</span></span>;</span><br><span class="line"></span><br><span class="line"><span class="comment">//指针和赋值动作可以分开</span></span><br><span class="line"><span class="function">shared_ptr&lt;User&gt; <span class="title">sp3</span><span class="params">(<span class="literal">nullptr</span>)</span></span>;</span><br><span class="line">sp3 = <span class="built_in">make_shared</span>&lt;User&gt;(<span class="number">3</span>, <span class="string">&quot;sp3&quot;</span>, <span class="string">&quot;123456&quot;</span>);</span><br><span class="line"></span><br><span class="line"><span class="comment">// Control Block的ref将会加一</span></span><br><span class="line">audo p4 = sp1;</span><br><span class="line"></span><br><span class="line"><span class="comment">//Control Block和Object都会被交换</span></span><br><span class="line">sp1.<span class="built_in">swap</span>(sp2);</span><br></pre></td></tr></table></figure>

<p>参考：<a target="_blank" rel="noopener external nofollow noreferrer" href="https://learn.microsoft.com/zh-cn/cpp/cpp/how-to-create-and-use-shared-ptr-instances?view=msvc-170">创建和使用 shared_ptr 实例</a></p>
<p>注意：不要使用裸指针来初始化智能指针，会导致多次释放内存造成崩溃：</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="type">void</span> <span class="title">test</span><span class="params">()</span> </span>&#123;</span><br><span class="line">    User* user = <span class="keyword">new</span> <span class="built_in">User</span>(<span class="number">0</span>, <span class="string">&quot;Jone&quot;</span>, <span class="string">&quot;12345&quot;</span>);</span><br><span class="line">    <span class="function">shared_ptr&lt;User&gt; <span class="title">p1</span><span class="params">(user)</span></span>;</span><br><span class="line">    <span class="function">shared_ptr&lt;User&gt; <span class="title">p2</span><span class="params">(user)</span></span>;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>当test()函数退出时，p2会先调用User析构，然后p1会再次调用User析构函数，同时两次释放同一块内存，就会导致程序出错。下面是正确的写法：</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="type">void</span> <span class="title">test</span><span class="params">()</span> </span>&#123;</span><br><span class="line">    <span class="function">shared_ptr&lt;User&gt; <span class="title">p1</span><span class="params">(<span class="keyword">new</span> User(<span class="number">0</span>, <span class="string">&quot;Jone&quot;</span>, <span class="string">&quot;12345&quot;</span>))</span></span>;</span><br><span class="line">    shared_ptr&lt;User&gt; p2 = p1;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

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